Saturn Helium Abundance: A Reanalysis of Voyager Measurements

Thermal emission spectra calculated using Voyager Jupiter radio occultation (RSS) temperature profiles rescaled to the Galileo probe value of the helium abundance do not agree with the spectra measured by the Voyager infrared spectrometer (IRIS). The ∼2 K offset in brightness temperature suggests the possibility of a systematic error source yet to be identified. This raises the question of the validity of the Voyager Saturn helium abundance that was determined using the same RSS–IRIS approach. We address this issue by developing an inversion algorithm for the simultaneous retrieval of the temperature, the para H 2 fraction, and the helium abundance from the IRIS spectra alone. This approach can not be successfully applied to Jupiter because of strong gaseous NH 3 and cloud opacity near the low-frequency end of the spectrum, but this restriction is less severe at the lower temperatures of Saturn. Applications of the algorithm to Saturn spectra yield a volume mixing ratio He/H 2 between 0.11 and 0.16 corresponding to a helium mass fraction relative to the total helium and hydrogen in Saturn's atmosphere of Y=0.18–0.25. Although these retrievals depend on subjective filtering of the solutions in the inversion algorithm to reduce the range of non-uniqueness for the helium values, they strongly suggest a value for He/H 2 significantly larger than the value of 0.034±0.024 previously obtained by Conrath et al. (Conrath, B. J., D. Gautier, R. A. Hanel, and J. S. Hornstein 1984, Astrophys. J. 282, 807–815) using the RSS–IRIS method.